1. Field of the Invention
This invention relates to a process for regenerating wastepaper and to the use of cationic layer compounds precipitated in situ for regenerating wastepaper.
2. Discussion of Related Art
"Deinking" is the removal of printing inks from wastepaper fiber suspensions. Deinking processes essentially comprise two steps, namely:
1. refining the wastepaper, i.e. fiberizing in water in the presence of the chemicals required for detachment of the printing ink particles and
2. removal of the detached printing ink particles from the fiber suspension.
The second step can be carried out by washing or flotation (Ullmanns Encyclopadie der technischen Chemie, 4th Edition, Vol. 17, pages 570-571 (1979)). In flotation, which utilizes the difference in wettability between printing inks and paper fibers, air is forced or drawn through the fiber suspension. Small air bubbles attach themselves to the printing ink particles and form a froth at the surface of the water which is removed by savers.
The deinking of wastepaper is normally carried out at alkaline pH values in the presence of alkali metal hydroxides, alkali metal silicates, oxidative bleaches and surfactants at temperatures in the range from 30.degree. to 50.degree. C. Soaps and/or fatty alcohol polyglycol ethers are often used as surfactants which are responsible for the detachment and separation of the printing inks (Ullmanns Encyclopadie der technischen Chemie, 4th Edition, Vol. 17, pages 571-572 (1979)).
In recent years, conventional printing ink systems, for example based on nitrocellulose, maleate resins and/or shellac, which contain esters and/or ketones, for example ethyl acetate and/or methyl ethyl ketone, or alcohols as solvent, have been increasingly replaced by water-dilutable printing inks for reasons of pollution control. Another reason for the increasing use of water-dilutable printing inks lies in the non-inflammability of water which eliminates the need for the expensive safety systems which normally have to be installed in the printing works where solvent-containing printing inks are used. Most water-dilutable printing inks contain as binder component anionic polymers, for example polymers containing carboxyl groups, of which the neutralization with bases makes the printing inks dilutable with water. However, water-dilutable printing inks have the major disadvantage that they can only be removed completely inadequately, if at all, with the surfactants typically present in the deinking liquor (Das Papier 42, V 84-V 88 (1988)). The result of this is that, hitherto, the wastepaper printed with water-dilutable printing inks, which is accumulating in ever-increasing quantities, is not recycled and, hence, is also not available as a wastepaper raw material for newsprint and sanitary papers.
Through the increasing constriction of water circuits in the paper industry, there has been a considerable increase in the concentration of ionic and/or nonionic, inorganic and/or organic substances soluble and/or colloidally dissolved in water. These substances, which are known as "trash" in the paper industry, on the one hand pollute the wastewater and, on the other hand, adversely affect paper manufacture, paper quality and the effectiveness of cationic auxiliaries (Wochenblatt fur Papierfabrikation 1984, 37-48). Starch, casein, polyvinyl alcohol, polymer dispersions, carboxymethyl celluloses and/or printing ink binders are examples of such trash.
The problem addressed by the present invention was to provide a process for regenerating wastepaper with which it would be possible to remove printing inks and, more particularly, water-dilutable printing inks, fillers and/or trash from fiber suspensions.
Cationic layer compounds, which are also referred to in the literature as "double-layer hydroxides" (R. Allmann "Doppelschichtstrukturen mit brucitahnlichen Schichtionen . . . " in Chimia 24, 99-108 (1970)) may be characterized by the following general formula EQU M(II).sub.1-x M(III).sub.x (OH)).sub.2 (A.sup.z-).sub.x/z .multidot.n H.sub.2 O
in which M(II) represents at least one divalent metal cation, M(III) represents at least one trivalent metal cation and A.sup.z- represents anions of monobasic and/or polybasic acids, x is a number of 0.01 to 0.5, n is a number of 0 to 20 and z corresponds to the number of charges of the anions. According to DE-OS 20 61 156, these layer compounds may be prepared by various methods, for example by adding an aqueous solution containing divalent and trivalent metal cations in the molar ratio represented by x in the above formula as salts of the desired acid H.sub.z A with stirring to an alkali metal hydroxide, preferably sodium hydroxide, at a pH value in the range from 9 to 12. The layer compound formed is removed, washed with water and dried. A well-characterized representative of this class of substances is hydrotalcite which occurs as a mineral in nature or may even be synthetically produced and which is a magnesium-aluminium hydroxocarbonate having the approximate composition EQU Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4 H.sub.2 O
of which the structure has been determined by X-ray photography (R. Allmann and H.P. Jepsen, "Die Struktur des Hydrotalcits" in N. Jahrb. Mineral. Monatsh. 1969, 544-551). However, printing inks and also fillers and/or trash cannot be satisfactorily removed from paper fibers with these dried and powdered cationic layer compounds. Poor results in the removal of printing inks, fillers and/or trash from wastepapers are also obtained where paste-like cationic layer compounds of the above composition which may be produced in accordance with DE 38 38 532 and which have a water content of at least 30% by weight, as determined after drying for 4 hours at 110.degree. C., are used.
However, it has surprisingly been found that printing inks, more particularly water-dilutable printing inks, and also fillers and/or trash can be effectively removed from fiber suspensions with cationic layer compounds of the above composition which are precipitated in situ in the fiber suspensions.